1. Field of the Invention
The present invention relates to wireless communication systems. More specifically, the present invention relates to systems and techniques for securing wireless communication systems such as spread spectrum systems.
2. Description of the Related Art
Security is a key issue in wireless communication systems such as cellular telephone systems, satellite communication systems and two-way radio links. Whether communication of voice or data is required and whether for military, commercial or consumer use, it is imperative that the link be secured from eavesdroppers or interferers whether intentional or otherwise.
Spread spectrum technology was developed to improve the security of wireless communication systems. In spread spectrum systems a carrier signal, which modulates input data, is moved or hopped over a band or spectrum of frequencies. A frequency hopping system spreads the energy of a transmitted signal across a bandwidth larger than the minimum required for the signal. The frequency hopping occurs many times per second. The transmitter and receiver must stay synchronized and hop to the same frequency at the same time.
A spread spectrum system offers some measure of security inasmuch as an eavesdropper or an interferer must know the frequency hopping sequence and hop in unison with the transmitter or the receiver of the system. If an eavesdropper does not know the frequency sequence, detection or interference become more difficult to achieve. Nonetheless, if the frequency hopping sequence repeats, detection of the frequency hopping sequence becomes possible and the security of the system is limited accordingly.
Pseudo-random frequency hopping sequences are used to prevent detection of a repeating frequency hopping sequence. These systems use a pseudo-random number generator to produce a substantially random sequence of hopping frequencies. This sequence is provided to both the transmitter and the receiver in advance of transmission. To the eavesdropper, the hopped signal appears to be low intensity noise spread over the entire bandwidth.
Unfortunately, however, since the output of a pseudo-random generator is "pseudo-random" and not "random", the output of a pseudo-random generator repeats. Accordingly, the degree to which these systems are secure is limited. Hence, there is an ongoing need in the art for systems and techniques to improve the security of wireless communication systems.